In wireless communication systems, data transmission from multiple antennas can be “precoded” by applying different weights on a signal to be sent from a set of transmit antennas. There can be one or more spatial “layers” of distinct data streams, up to the number of transmit antennas. The precoding operation can be represented mathematically by a precoder, or precoding matrix that includes a vector form as the special case. The precoder matrix transforms or maps the one or more data layers to the transmit antennas. Precoding transmission can happen in either downlink (i.e., from a base station to a device) or in uplink (i.e., from a device to a base station). In uplink precoding, a base station typically needs to instruct the device to use a precoding matrix selected from a set of precoding matrices known to both base station and the device, the set referred to as a codebook. Precoding can be used to obtain beamforming gain by transmitting a single spatial layer, or improve spectral efficiency using spatial multiplexing, where two or more layers are transmitted simultaneously. Further, the one or more layers may be obtained from one or more “transport blocks”. A Transport Block (TB) is used to represent a block of information bits which are uniquely coded into a codeword which is mapped to the corresponding one or more spatial layers. One transport block may be mapped to one or more layers, but a layer corresponds to only one transport block.
In the case of two or more transport blocks set simultaneously using an indicated precoding matrix, sometimes not all transport blocks can be decoded successfully. In that case, the failed transport block needs to be transmitted again later (known as Hybrid ARQ or HARQ retransmission) and there may be no new transport block to be sent at the same time in place of the successful transport blocks. Since the total number of spatial layers is reduced in this case, the precoding matrix also needs to change. The HARQ retransmission may often occur based on predefined protocol without any explicit instruction of the new precoding matrix from the base station. This invention describes methods that determine the new or retransmission precoding matrix that a device can use when retransmitting the unsuccessful transport block(s), using a combination of techniques.
The various aspects, features and advantages of the invention will become more fully apparent to those having ordinary skill in the art upon a careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.